Hydraulic starter system for engines



Jan. 7, 1964 2 Sheets-Sheet l vFiled May 22, 1961 9164 NAS@ ,EN 30H l"illlllli ATTORNEYS Jan- 7, 1964 M. 1 KENT ETAL HYDRAULIC STARTER SYSTEMFOR ENGINES 2 Sheets-Sheet 2 Filed May 22. 1961 ATTORNEYS United StatesPatent O 3,116,595 HYDRAUMC STARTER SYSEM FR ENGNES Melvin L. Kent, EastCleveland, (Ehio, and Mitre lirewarsld, Bridgeport, Conn., assignorsto'The New Yorlr Air Brake Company, a corporation of New llersey FiledMay 22, 196i., 'Ser'. No. llL'Zl 5 Claims. (Cl. Gil- 18) This inventionrelates to a hydraulic starting and pumping system.

The use of hydraulic motors for starting internal combustionvehicle-propulsion engines is known in the prior art. Such hydraulicmotors are generally driven by pressure fluid supplied by motor-drivenauxiliary pumps and the known systems have -proven to be complex, heavy,space-consuming and expensive.

In the copending application of Edward V. Manning and lohn P. Mentink,Serial No. 92,052, led February 27, 1961, and entitled Hydraulic System,a hydraulic starting system is disclosed in which a variabledisplacement hydraulic motor-pump unit is driven Las a motor by Huidsupplied from a pressure tluid source, such as a pressure accumulator,having a. decaying pressure characteristic. Automatic control means areprovided for controlling the displacement of the hydraulic unit in apredetermined manner in accordance with the decaying characteristic ofthe source. By the use of such a pressure fluid source, the startingsystem is capable of being completely self-contained in the vehicle.After the internal combustion engine has been started, the hydraulicunit is designed to be driven as a pump by the internal combustionengine to recharge the accumulator and to supply fluid to the hydraulicsystem of the vehicle. By causing the hydraulic motor of the startingsystem to operate also as a pump subsequent to starting of the internalcombustion engine, the need for auxiliary pumping apparatus iseliminated and the weight of the vehicle is reduced.

The object of the present invention is to provide an improved startingand pumping system of the type disclosed in the Manning and Mentinkapplication referred to above. The invention is characterized by theprovision of means operable-during motoring-to position the displacementcontrol element of the variable displacement unit in a maximumstroke-establishing motoring position as long as the pressure of thesource is above a predetermined value and to position `the controlelement in a zero stroke-establishing neutral position when the pressureis below this value, and operable-during pumpingto position the controlelement in a maximum stroke-establishing pumping position when thepumping pressure of the unit is below a predetermined operating valueand to move the control element gradually toward its neutral position asthe pumping pressure rises from this operating value to the desiredmaximum.

The hydraulic motor-pump unit of the starting and pumping system is ofthe overcenter type employing a rotary group of elements consisting of acylinder barrel havinor longitudinal bores therein and reciprocatorypistons mounted in said bores, said rotary group being mounted forrotation with respect to an angularly adjustable cam plate suspended inthe engine housing. As the rotary group of elements rotates relativelyto the cam plate, reciproeatory motion is imparted to the pistons. Thestroke of the pistons and the displacement of the hydraulic unit -aredependent upon the angle of inclination of the cam plate. The cam plateis movable between maximum stroke-establishing motoring and pumpingpositions on opposite sides of a zero stroke-establishing neutralposition. Such a hydraulic unit presents the advantage that only onehigh pressure line is required since the high pressure inlet port of thehydraulic unit during motoring becomes the high pressure discharge portduring pumping.

Patented Jan. '27, 1%64 'ice According to the preferred embodiment ofthe invention, a shiftable spring assembly is provided for continuouslybiasing the displacement control member toward its maximumstroke-establishing pumping position during the motoring and pumpingoperations of the hydraulic unit. A shifting motor is provided forplacing the spring assembly, during motoring, in a rst position in whicha given biasing force is applied to the control member, and for placingthe spring assembly, during pumping operation, in a second position inwhich a greater biasing force is applied to the control member.Pressure-responsive means are provided for placing the control member inits maximum stroke-establishing motoring position (against thecounteracting bias of the spring assembly) when the pressure of thesource is above a predetermined value. During pumping operation, thepressure-responsive means operate to position the control member in itsneutral position (against the counteracting bias of the spring assembly)when the pumping pressure equals a predetermined operating value.According to another feature of the invention, the spring assembly isdesigned to constitute a stop which operates, during pumping operationof the hydraulic unit, to prevent movement of the control element fromits neutral position toward its maximum strokeestablishing motoringposition.

Other obiects and advantages of the invention Will become more apparentfrom a study of the following specification when considered inconjunction with the accompanying drawings, in which:

FlG. l is a sectional view of the hydraulic unit of the starting system,the displacement control member being shown in its maximumstroke-establishing motoring position.

FIG. 2 is a schematic diagram of the hydraulic starting system, thedisplacement control element of the hydraulic unit being shown in itszero stroke-establishing neutral position.

The hydraulic starting system includes a variable displacement hydraulicmotor-purnp unit 3- of the overcenter type disclosed in detail in thecopending application of Tadeusz Budrich and Edward V. Manning, SerialNo. 789,996, tiled January 29, 1959, now abandoned, and entitledOvercenter Hydraulic Starter Pump. The hydraulic unit comprises ahousing 4 having a drive shaft 5 journalled therein to which isconnected the rotary cylinder barrel 6. The rotary cylinder barrel 6 isin contiguous engagement at one end with the adjacent end face ofstationary valve plate '7, which face contains arcuate high and lowpressure ports in communication, respectively (through internal housingpassages, not shown), with the housing high and low pressure ports 8 and9 illustrated in FlG. 2. The cylinder barrel 6 contains acircumferential series of through longitudinal cylinder bores l1 whichare arranged to register sequentially with the arcuate ports in valveplate 7 as the cylinder barrel rotates. Pistons l2, formed withspherical heads 13 for connection with supporting shoes ld, are mountedin the cylinder bores for reciprocation by cam plate l5 and return platelo.

The cam plate l5' is supported in housing 4- by cam plate yoke portionsi551 and fixed housing-secured trunni-ons l' (see FiG. 2) for angularmovement about ya horizontal pivot axis extending in a directionalnormal to and intersecting the vaxis of drive shaft 5. The angularposition of cam plate 15 determines the len-gth of the strokes ofpistons l2, and the cam plate is free to move between maximumstroke-establishing moto-ring (FIG. 1) tand pumping positions onopposite sides of a zero strokeestablishing neutral (FIG. 2) position.Shiftable spring assembly llii is provided for biasing cam plate 15 inthe clockwise direction about its pivot axis. This spring assemblycomprises -a iirst spring seat sleeve 19 connected at one end with thecam plate 15 by connecting rod 2,1,

a second spring seat sieeve 22 arranged for sliding movement at one endwithin an axial bore in the other end of sleeve i9, and a coaxialprcloaded coil spring 23 seated on the per' hcral liange portions and22a of sleeves i9 and 22, respectively. When sleeve 22 is in its FIG. 1position in engagement with the housing inner wall surface 4a, Spring 23reacts with a first biasing force between sleeve portions Na and 22a todisplace sleeve 19 to the left and thereby piace cam plate in itsmaximum stroke-establishing pumping position as shown by the brokenlines in FIG. 2.

Motor means 24 are provided for shifting sleeve 22 to the left to asecond position in which spring 23, during pumping operation of thehydraulic unit, biases cam plate h toward its maximumstroke-establishing pumping position with a greater biasing force. Thismotor 24 includes Aa cylinder 25 provided with an axial tubularextension 25a secured within an opening in the housing 4. Reciprocablepiston 26 is mounted in cylinder 25 and is provided with a piston rod 27movable within extension 25a. The second sleeve 22, which is mounted forsliding movement on this stationary extension 25a, has an end wall 22biagainst; which the free end of piston rod 27 abuts. When pressure fluidis introduced into working chamber 24u of motor 24 to displace piston 26to the left into engagement with the left-hand end wall of cylinder 25,piston rod 27 shifts sleeve 22 to the left and thereby increases thespring biasing force on cam plate 15. It is important to note that whensleeve 22 is in this left-hand position, it serves as a cam stoppreventing cam plate 15 from moving from its neutral position toward itsFIG. 1 position.

Cam plate l5 is movable in the counterclockwise direction against thecounteracting biasing force of spring assembly 18 by a hydraulicpositioning motor 31 including a cylinder 32 secured to housing 4 and areciprocable piston 33 that is connected with cam plate 15 by connectingrod 34. Piston 33 is moved to the left in its cylinder by theapplication of pressure huid to working chamber 31a.

The drive shaft 5 extends through an opening in the housing 4 andterminates in a splined coupling 36 to which the internal combustionengine 37 is connected via conventional mechanical coupling 38.

Referring now to FiG. 2, an accumulator 40, which constitutes a pressurefluid source having ia decaying pressure characteristic, is connectedwith the housing high pressure port 8 via conduit containing three-wayinlet valve 42. Valve y42 is operable between a first position in whichit connects conduit portion 41a with conduit portion Elib and a secondposition in which it connects conduit portion 4th with the vehiclehydraulic system conduit Housing low pressure port 9 is connected withsump dfi via conduit d5.

The state of energization of positioning motor 3i is controlled by thedual-pressure control valve 46. This control valve inoludes a housinghaving an inlet passage 47 connected with high pressure port S viaconduit a motor passage 4E-9 connected with positioning motor workingchamber 33;'[1 via conduit 5E, `and an exhaust passage 52 connected withsump d5' via conduit 53. The control valve housing includes alongitudinal bore 54 in one `end of which is mounted the reciprocablevalve plunger 55 having lands 56, 57 'and 58 separated by grooves S9 and6l, respectively, for controlling the communication between the variousvalve housing passages. Through longitudinal slots @52 are provided inthe outer periphery of land 58.

Mounted for reciprocation in the other end of the valve housing bore isthe spring seat 63. Preloaded spring 64, reacting between spring seat 63and plunger 5S, urges the seat to the left to its FG. 2 position inwhich it abuts housing wall projection 65, and urges the plunger 55 inthe opposite direction into engagement with the other bore end wall toei"ect venting of the Working chamber 31a of positioning motor 31 viaconduit 5l, passage 49,

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groove 59, passage 52 and conduit Spring chamber 65 is in continuouscommunication with sump conduit 53 via conduit 67.

F1/hen seat is in its FG. 2 position, spring 64 establishes ia iirstreference pressure condition of Operation of control valve `as will bedescribed in greater detail below. Spring seat 63 constitutes the pistonof hydraulic piston motor means and, when pressure fluid is introducedinto working chamber 65a to shift the seat to the right to a secondposition in which it engages fixed stop 69, the preloading of spring6dis increased to establish a second reference pressure condition ofoperation of control valve The spring assembly shitting motor 24 and themotor for shifting spring seat 63 are operated simultaneously byauxiliary valve 71. This auxiliary valve includes a housing having aninlet port 72 connected with high pressure port S via conduit '73 andconduit portion 48a, a motor port connected with working chamber 24a viaconduits 75a 'and 'Sb and with working chamber 63a via conduits 75a and75e, and a pair of exhaust ports 7o and 'i7 connected with sump 44 viaconduits '78 and 79, respectively. The valve housing contains alongitudinal bore in which is mounted the movable valve plunger Sihaving lands S2, 83 `and 34 separated by grooves S5 and S6. Spring S7,mounted in the left-hand end of the bore, biases plunger 31 to the rightto a first or energizing position in which working chambers 24a and 63acommunicate with high pressure port S via branch portions '/"Sb i idF.Jc, respectively, common portion 75a, port 7d, groove 85, port '72,conduit 73 and conduit portion 43a. Plunger Si carries an 'axialprojection da on which is secured the armature SS of stationary solenoidcoil S9. When solenoid coil 89 is energized by the closing of switch 9E,armature 88 is displaced to the left by magnetic attraction to moveplunger 81 to the left to its second `or venting position, shown in FIG.2, in which working chambers 24a and 63a `are connected with sump viabranch line portions 75h `and 75C, respectively, common portion 75a,port 74, groove S6, port '77, and conduit '79.

OPERATION The operation of the hydraulic starting system may now bedescribed.

(l) Motoring Operation Assuming that the hydraulic unit 3 is at rest andthat valve 42 is in its second position, the tluid path from accumulatorfat) to high pressure port 8 is closed and the initial pressures in port8 and control valve inlet chamber 47 are zero. Plunger 55 is maintainedin its FiG. 2 lposition by spring 64 so that Working chamber Sia otpositioning motor 31 is vented to sump via conduit 5E, passage 49,groove 59, passage 52 and conduit 53. suming also that switch 9i isclosed to energize solenoid 89, plunger 8i is maintained in .its FIG. 2position against the bias of spring 37 to connect Working chambers fiaand 63a with sump 44, as shown. Since working chamber Zita is vented,spring seat sleeve 22 engages the housing internal wall surface 4a andspring 23 biases sleeve 19 to the left relatively to sleeve 22 toposition cam plate l5 in its maximum stroke-establishing pumpingposition, as shown by the broken lines in FiG. 2. Owing to the ventingof working chamber 63a, seat 63 is in its PEG. 2 position to establishthe rst reference pressure value against which the pressure in inletpassage 47 is compared.

For the purpose of the `following discussion, it will be assumed thatthe first reference pressure established by spring 64 is 1500 psi., thatthe accumulator 40 is under an initial pressure of 3000 p.s.i., and thatpressures of 300 and 500 psi., respectively, in working chamber 3M arerequired to hold cam plate 15 in the neutral position of FIG. 2 and themaximum motoring position of FIG. 1 against the bias of spring 23 whenchamber 24a is vented.

When inlet valve 42' is opened, pressure iluid is transmitted to port S,through conduit 41 and to inlet passage 47 yof control valve -46 viaconduit 4S. The fluid in passage 47 acts on the right-hand end surfacesof lands 57 and 5S and shifts plunger 55 to the left against the bias ofspring 64. As the pressure rises to the reference value of 1500 p.s.i.,plunger 55 moves to a lap position in which land 57 interruptscommunication between motor passage 49 and exhaust passage 52. A furtherrise in press-ure in inlet passage 47 lcauses plunger 55 to move furtherto the left to yinterconnect inlet passage 47 with motor passage 49through slots 62 and .groove 61. Pressure iluid is now transmitted toworking chamber 31a through conduit 51. The pressure in working chamber31a rises rapidly to system pressure, and since at this time thispressure is greater than the 500 p.s.i. required to hold carn plate 15in the maximum motoring position of FIG. l, the motor 311 immediatelymoves the cam plate 15 to that position. The cam plate 15 remains inthis position as long as system pressure is above 1500 p.s.1.

The iiuid supplied to high pressure port 8 from the accumulator 4i) isfed through internal housing passages (not shown) and through thepassages of valve plate 7 to the longitudinal bores 11 of the rotarycylinder barrel 6 to displace pistons 12 to the left against cam plate15. The pistons react with the inclined cam plate to apply maximuminitial torque on drive shaft with the result that internal combustionengine 37' is driven in the starting direction by the hydraulic unit 3through the splined coupling 36 and the mechanical connection 38.

As the speed of rotation of the hydraulic motor 3 increases, it imposesan increasing demand on the accumulator 40 and consequently the supplypressure decreases progressively. For the purpose of the followingdiscussion, it is assumed that by the time the accumulator pre sure hasdecreased to 1490 p.s.i., the internal combustio-n engine hasaccelerated to starter cut-out spe-ed. As the hydraulic unit 3accelerates to approximately 93 of starter cut-out speed, theaccumulator pressure decreases from 3000 to 1600 p.s.i. and a similarpressure reduction occurs in working chamber 31a. Throughout this periodof time, the pressure in working chamber 31a is sufficient to maintaincam plate v15 in its maximum stroke-establishing motoring positionagainst the bias of spring '23. Consequently the hydraulic motorcontinuous- 1y applies to the internal combustion engine 37 the maximumpossible torque obtainable from the lluid pressure source. During thefinal period of acceleration o'f the hydraulic unit, ie., as the speedof the internal combustion engine increases from 93% to 100% of startercutout speed, the accumulator pressure drops below the referencepressure of 1500 psi. thus permitting spring 64 to move plunger 55 tothe right from the lap position and again vent working chamber 31a. Asthe pressure in working chamber 31a decreases below 500 p.s.i., spring23 commences to move cam plate 15 toward the neutral position, and, whenit reaches 300 p.s.i., the cam plate will be in that position.

As the pressure in working chamber 31a approaches 300 p.s.i., theinternal combustion engine approaches starter cut-out speed, and, whenthe working chamber pressure is 300 p.s.i., the engine will haveachieved independent operation and will be driving hydraulic unit 3 as apump. Because of this, the pressure -in conduit 48, and in inlet passage47, is immediately restored to 1500 p.s.i. and the plunger 55 of controlvalve 45 is return-ed to the lap position thereby interrupting the ventpath from working chamber 31a. Since there is no demand for hydraulicuid at this time, system pressure will remain at 1500 psi. and controlvalve 46 will maintain cam plate 15 in neutral position. However, shouldsystem pressure decrease as a result of leakage, spring ed will shiftplunger 55 to the right from the lap position to thereby vent Workingchamber 31a and permit spring 23 to move cam plate 15 away from neutralposition a small distance in the direction of maximum pumping position.This action causes the pump to supply iluid to the system and when thepressure is restored to 1500 p.s.i. and the rate of discharge from thepump equals the rate of leakage, plunger 55 will be shifted back to thelap position. It should be realized that the cam plate positionestablished by system leakage is very close to the neutral position andthat for all practical purposes, the hydr-wlic unit is now imposing aminimum torque on the internal combustion engine. This is a `desirablefeature because it permits the engine to accelerate rapidly.

(2) Pumping Operation After the internal combustion engine hasaccelerated to a speed at which it develops sucient power to drive thehydraulic unit under load, switch 91 is opened to initiate the highpressure pumping operation. Opening of switch 91 de-energizes theholding solenoid 3% and allows spring 87 to move valve plunger v81 tothe right to connect working chambers 24a and 63a with high pressureport 3 via branch conduits 75h and 75C, respectively, common conduit75a, port 74, groove 85, port 72, conduit 73 and conduit portion 48a.The pressure Huid in working chamber 24a moves piston 26, piston rod 27,and sleeve 22 to the left to increase the biasing force applied to camplate 15 by spring 23. lt is assumed that with sleeve 22 in itsleft-hand position, a pressure of p.s.i is required in chamber 31a toinitiate movement of cam plate 15 in the counterclockwise directionagainst the bias of spring Z3, and that a pressure of 350 p.s.i. isrequired to position the cam plate in its neutral position. Sleeve 22now constitutes a stationary cam stop preventing movement of the camplate from its neutral position toward its maximum stroke-establishingmotoring position.

The pressure uid in working chamber 63a moves spring seat 63 to theright into engagement with stop 69 with the result that the compressionload in spring 64 is increased to establish a second eference pressurevalue (for example, 3000 p.s.i.). Plunger 55 is now in its FlG. 2position, venting chamber 31a.

With cam plate 15 in its maximum stroke-establishing pumping position,the hydraulic unit is driven by the internal combustion engine andsupplies lluid from high pressure port 8 to the accumulator d0 viaconduit 41. After the accumulator has been recharged .to its initialpressure of 3000 p.s.i., which is the second reference pressure, plunger55 is shifted to the left to its lap position. When the pressure exceeds3000 p.s.i., plunger 55 is shifted further to the lleft to admit uid tochamber 31a via slots 62, grove 61, chamber 49 and conduit 51. As thepressure in chamber 31a increases from 150 p.s.i. to 350 p.s.i., camplate 15 is progressively pivoted from its maximum stroke-establishingposition to its neutral position. Due to the provision of the cam stop(i.e., sleeve 22 in its left-hand position), movement of the cam platein the counterclockwise direction beyond the neutral position isprevented. Since the cam plate is now in its neutral position, thehydraulic unit is in an idling, minimum-displacement condition. in theevent that the pressure in conduit 4?, drops below 3000 psi. (as aresult of internal leakage, for example), plunger 55 is shifted to theright by spring 64 to vent chamber 31a, and when the pressure in thischamber drops below 350 p.s.i., cam plate 15 is displaced in theclockwise direction by spring 23 to effect an increase in displacementof the pump to compensate for the leakage. When the pressure in conduit4S is returned to 3000 p.s.i., plunger 55 is moved to the left to thelap position to trap the fluid in chamber 31a, whereby cam plate 15 ismaintained in its leakage compensating position.

inlet valve 42 may now be shifted to its second position in whichaccumulator itl is isolated and high pressure port S is connected withthe vehicle hydraulic system via conduits fili; and As a result oisystem demand, the pressure in conduit 4S and passage i7 decreases below300() p.s.i. and plunger 55 is shifted to the right to vent workingchamber 3fm. As the pressure in chamber 3M decreases from 350 to 150p.s.i., cam plate i5 is pivoted in the clockwise direction toward itsmaximum strokeestablishing pumping position by spring 23. When thesystem demand is met by the pump, the pressure in conduit 48 rises tothe second reference pressure value vand plunger 5S is shifted to theleft to its lap position to discontinue venting of chamber 31a. Theposition of the cam plate i5 now establishes a rate of discharge equalto the rate of demand for hydraulic iluid.

Should the system demand decrease and the pressure rise above 3000p.s.i., plunger 55 is shifted further to the left to supply lluid tochamber 32a, and thus cause motor Si to eiect a progressive decrease incam angle. When the cam reaches that position in which the rate ofdischarge from the hydraulic unit 3 matches the new demand and systempressure is restored to 3000 p.s.i., plunger 55 moves back to its lapposition and again interrupts ow to working chamber 31a. Thus it isapparent that during pumping operation, the hydraulic unit operates as apressure-compensated pump, the displacement of which is automaticallyvaried by valve 46 to maintain substantially constant the selectedmaximum system pressure.

As stated previously, the drawings and description relate only to thepreferred embodiment of the invention. Since many changes can be made inthe structure of this embodiment without departing from the inventiveconcept, the following claims should provide the sole measure of thescope of the invention.

What is claimed is:

l. In combination (a) hydraulic motor-pump unit having high and lowpressure ports and a displacement control element movable between firstand second maximum displacement-establishing positions on opposite sidesof a Zero displacement-establishing position ifor Varying thedisplacement of and changing the direction of ow through the unit;

(b) a source of pressure fluid, having a decaying pressurecharacteristic, for driving the unit as a motor;

(c) a conduit connecting the source with the high pressure port;

(d) a reservoir connected with the low pressure port;

(e) spring means connected with the displacement control element andbiasing it toward the second maximum displacement-establishing position;

(f) control means connected with the displacement control element andresponsive to the pressure at the high pressure port for moving saidelement toward the rst maximum displacement-establishing position lasthe pressure rises above a predetermined vaine;

(g) stop means associated with the displacement control element andshiftable between effective and ineiective positions in which,respectively, it prevents and permits said element from moving from thezero displacement-establishing position toward the rst maximumdisplacement-establishing position; and

(lz) selecting means for sluiting the stop means between its eective andineffective positions.

2. The combination defined in claim l (a) in which the spring meansincludes a spring seat shiftable between high spring-load and lowspringload positions; and

(b) in which the seat and the stop means are interconnected, whereby theseat is shifted to the high spring-load and low spring-load positions,respectively, as the stop means is shifted to the elective .andineffective positions.

3. The Combination defined in claim 2 (a) in which the control meanscomprises (l) a iluid pressure positioning motor connected with thedisplacement control cient-nt and arranged to move that element towardthe Erst maximum displmoment-establishing position, (2) a control valveconnected with the positioning motor, the high pressure port and thereservoir and including a movable member shittable between a iirst p.sition in vhic'n the positioning motor is co s leeted with the reservoirand a second position in which thc positioning motor is connected withthe high pressure port, the movable member ha g an intermediate lapposition in which the posaioning nioter is isolated from both thereservoir and the high pressure port,

(3) means biasing the valve member toward the first (4) means responsiveto the pressure pressure port for shifting the valve toward the secondposition, and

(5) means form varying the bias exerted by tlfc biasing means betweenhigh and low values; and

(b) in which the seiccting includes rneans for operating the biasvarying means simultaneously with shifting of the stop means and thespring seat, whereby the bias exerted by the biasing means is said lowvalue when the stop means is in the ineiective position and the springseat is in the low spring-load position, and the bias exerted by thebiasing means is said high value when stop means and the spring seat arein their other positions.

4. In combination (a) a hydraulic motor-pump unit having high and lowpressure ports and a displacement control ent movable between first andsecond maximum displacement-establishing positions on opposite sides ofa zero displacement-esta'olishing position for varying the displacementof and changing the direction of flow through the unit;

(b) a source of pressure fluid, having a decaying pressurecharacteristic, for driving the unit as a motor;

(c) a conduit connecting the source with the high pressure port;

(d) a reservoir connected with the low pressure port;

(e) spring means connected with the displace nent control elernent andbiasing it toward the second maximum displacernent-establishingposition,

(f) said spring means including a spring seat shiftable between highspring-load and lov.l spring-load positions;

(g) a fluid pressure shifting motor connected with the spring seat andarranged to shift the spring scat toward the high sprinU-load position;

(lz) selecting means for connecting the shifting motor with the highpressure port and the reservoir, respectively; and

(i) control means connected with the displacement control element andresponsive to the pressure at the high pressure port for moving saidelement toward the lirst maximum displacement-establishing position asthe pressure rises above a predetermined value.

5. The combination defined in claim 4 (a) in which the control meanscomprises (l) a uid pressure control motor connected with thedisplacement control element and arranged to move that element towardthe first maximum displacement-establisiting position,

(2) a control valve responsive to the pressure at the high pressure portfor connecting the control motor with the 1righ pressure port and thereservoir, res ectively, as the pressure rises above and decreases belowsaid predetermined value, and

at the high niemeer 9 10 (3) uid pressure motor means associated withmotor and the fluid pressure motor means are sithe control Valve andeffective when pressurized multaneously connected with the high pressureport to establish a high limit for said predetermined and the reservoir,respectively. Value. and when ".ented to estabhsh a 10W 1mm ReferencesCited in the ille of this patent for said predetermined value; and 5 (b)in which the selecting means is connected with the UNITED STATES PATENTSfluid pressure motor means, whereby the shifting 2,986,872 Budzich June6, 1961

1. IN COMBINATION (A) HYDRAULIC MOTOR-PUMP UNIT HAVING HIGH AND LOWPRESSURE PORTS AND A DISPLACEMENT CONTROL ELEMENT MOVABLE BETWEEN FIRSTAND SECOND MAXIMUM DISPLACEMENT-ESTABLISHING POSITIONS ON OPPOSITE SIDESOF A ZERO DISPLACEMENT-ESTABLISHING POSITION FOR VARYING THEDISPLACEMENT OF AND CHANGING THE DIRECTION OF FLOW THROUGH THE UNIT; (B)A SOURCE OF PRESSURE FLUID, HAVING A DECAYING PRESSURE CHARACTERISTIC,FOR DRIVING THE UNIT AS A MOTOR; (C) A CONDUIT CONNECTING THE SOURCEWITH THE HIGH PRESSURE PORT; (D) A RESERVOIR CONNECTED WITH THE LOWPRESSURE PORT; (E) SPRING MEANS CONNECTED WITH THE DISPLACEMENT CONTROLELEMENT AND BIASING IT TOWARD THE SECOND MAXIMUMDISPLACEMENT-ESTABLISHING POSITION; (F) CONTROL MEANS CONNECTED WITH THEDISPLACEMENT CONTROL ELEMENT AND RESPONSIVE TO THE PRESSURE AT THE HIGHPRESSURE PORT FOR MOVING SAID ELEMENT TOWARD THE FIRST MAXIMUMDISPLACEMENT-ESTABLISHING POSITION AS THE PRESSURE RISES ABOVE APREDETERMINED VALUE; (G) STOP MEANS ASSOCIATED WITH THE DISPLACEMENTCONTROL ELEMENT AND SHIFTABLE BETWEEN EFFECTIVE AND INEFFECTIVEPOSITIONS IN WHICH, RESPECTIVELY, IT PREVENTS AND PERMITS AND ELEMENTFROM MOVING FROM THE ZERO DISPLACEMENT-ESTABLISHING POSITION TOWARD THEFIRST MAXIMUM DISPLACEMENT-ESTABLISHING POSITION AND (H) SELECTING MEANSFOR SHIFTING THE STOP MEANS BETWEEN ITS EFFECTIVE AND INEFFECTIVEPOSITIONS.